Wow. Well first I'll begin to say that the God of the Old Testament remains God today, which also means Jesus remains the same today as he did in the days of the Old Testament. The Triune God remains unchanged. Jesus came to fulfill the Old Testament promise of the Messiah, and salvation is from him. We say salvation comes through Israel, because Jesus came through Israel. Both Jews and Gentiles can accept Christ's gift of salvation. In addition, the ancient blessings for Israel stand.
But since you seem to recognize only the New Testament, please read these words from Romans 11:25-29. ...a partial hardening has happened to Israel until the fullness of the Gentiles has come in; and so all Israel will be saved, just as it is written, "the Deliverer will come from Zion, He will remove ungodliness from Jacob. This is My convenent with them, when I take away their sins. From the standpoint of the gospel they are enemies for your sake, but from the standpoint of God's choice they are beloved for the sake of the fathers; for the gifts and the calling of God are irrevocable.